Variables in C Topics Naming Variables Declaring Variables
Variables in C Topics • • Naming Variables Declaring Variables Using Variables The Assignment Statement Reading • Sections 2. 3 - 2. 4 CMSC 104, Lecture 12 1
What Are Variables in C? • Variables in C have the same meaning as variables in algebra. That is, they represent some unknown, or variable, value. x=a+b z + 2 = 3(y - 5) • Remember that variables in algebra are represented by a single alphabetic character. CMSC 104, Lecture 12 2
Naming Variables • Variables in C may be given representations containing multiple characters. But there are rules for these representations. • Variable names in C o o May only consist of letters, digits, and underscores May be as long as you like, but only the first 31 characters are significant May not begin with a number May not be a C reserved word (keyword) CMSC 104, Lecture 12 3
Reserved Words (Keywords) in C • • auto case const default double enum float goto CMSC 104, Lecture 12 break char continue do else extern for if int register short signed sizeof struct typedef unsigned volatile long return static switch union void while 4
Naming Conventions • C programmers generally agree on the following conventions for naming variables. o o o Begin variable names with lowercase letters Use meaningful identifiers Separate “words” within identifiers with underscores or mixed upper and lower case. Examples: surface. Area surface_area Be consistent! CMSC 104, Lecture 12 5
Naming Conventions (con’t) • Use all uppercase for symbolic constants (used in #define preprocessor directives). • Examples: #define PI 3. 14159 #define AGE 52 CMSC 104, Lecture 12 6
Case Sensitivity • C is case sensitive o o It matters whether an identifier, such as a variable name, is uppercase or lowercase. Example: area AREA Ar. Ea are all seen as different variables by the compiler. CMSC 104, Lecture 12 7
Which Are Legal Identifiers? AREA 3 D Last-Chance x_yt 3 num$ lucky*** CMSC 104, Lecture 12 area_under_the_curve num 45 #values pi %done 8
Declaring Variables • Before using a variable, you must give the compiler some information about the variable; i. e. , you must declare it. • The declaration statement includes the data type of the variable. • Examples of variable declarations: int meatballs ; float area ; CMSC 104, Lecture 12 9
Declaring Variables (con’t) • When we declare a variable o o o Space is set aside in memory to hold a value of the specified data type That space is associated with the variable name That space is associated with a unique address • Visualization of the declaration int meatballs ; meatballs garbage FE 07 CMSC 104, Lecture 12 10
More About Variables C has three basic predefined data types: • Integers (whole numbers) o int, long int, short int, unsigned int • Floating point (real numbers) o float, double • Characters o char • At this point, you need only be concerned with the data types that are bolded. CMSC 104, Lecture 12 11
Using Variables: Initialization • Variables may be be given initial values, or initialized, when declared. Examples: length int length = 7 ; 7 diameter float diameter = 5. 9 ; 5. 9 initial char initial = ‘A’ ; CMSC 104, Lecture 12 ‘A’ 12
Using Variables: Initialization (con’t) • Do not “hide” the initialization o o o put initialized variables on a separate line a comment is always a good idea Example: int height ; /* rectangle height */ int width = 6 ; /* rectangle width */ int area ; /* rectangle area */ NOT int height, width = 6, area ; CMSC 104, Lecture 12 13
Using Variables: Assignment • Variables may have values assigned to them through the use of an assignment statement. • Such a statement uses the assignment operator = • This operator does not denote equality. It assigns the value of the righthand side of the statement (the expression) to the variable on the lefthand side. • Examples: diameter = 5. 9 ; area = length * width ; Note that only single variables may appear on the lefthand side of the assignment operator. CMSC 104, Lecture 12 14
Example: Declarations and Assignments #include <stdio. h> inches int main( ) { int inches, feet, fathoms ; feet fathoms = 7 ; feet = 6 * fathoms ; inches = 12 * feet ; • • • CMSC 104, Lecture 12 garbage fathoms 7 feet 42 inches 504 15
Example: Declarations and Assignments (cont’d) • • • printf (“Its depth at sea: n”) ; printf (“ %d fathoms n”, fathoms) ; printf (“ %d feet n”, feet) ; printf (“ %d inches n”, inches) ; } return 0 ; • %d is a place holder - indicates that the value of the integer variable is to be printed in decimal form (rather than binary or hex) at that location. CMSC 104, Lecture 12 16
Enhancing Our Example • What if the depth were really 5. 75 fathoms? Our program, as it is, couldn’t handle it. • Unlike integers, floating point numbers can contain decimal portions. So, let’s use floating point, rather than integer. • Let’s also ask the user to enter the number of fathoms, rather than “hard-coding” it in. CMSC 104, Lecture 12 17
Enhanced Program #include <stdio. h> int main ( ) { float inches, feet, fathoms ; printf (“Enter the depth in fathoms : ”) ; scanf (“%f”, &fathoms) ; feet = 6 * fathoms ; inches = 12 * feet ; printf (“Its depth at sea: n”) ; printf (“ %f fathoms n”, fathoms) ; printf (“ %f feet n”, feet) ; printf (“ %f inches n”, inches) ; return 0 ; } CMSC 104, Lecture 12 18
Final “Clean” Program #include <stdio. h> int main( ) { float inches ; /* number of inches deep */ float feet ; /* number of feet deep */ float fathoms ; /* number of fathoms deep */ /* Get the depth in fathoms from the user */ printf (“Enter the depth in fathoms : ”) ; scanf (“%f”, &fathoms) ; /* Convert the depth to inches */ feet = 6 * fathoms ; inches = 12 * feet ; CMSC 104, Lecture 12 19
Final “Clean” Program (con’t) /* Display the results */ printf (“Its depth at sea: n”) ; printf (“ %f fathoms n”, fathoms) ; printf (“ %f feet n”, feet); printf (“ %f inches n”, inches); return 0 ; } CMSC 104, Lecture 12 20
Good Programming Practices • Place each variable declaration on its own line with a descriptive comment. • Place a comment before each logical “chunk” of code describing what it does. • Do not place a comment on the same line as code (with the exception of variable declarations). • Use spaces around all arithmetic and assignment operators. • Use blank lines to enhance readability. CMSC 104, Lecture 12 21
Good Programming Practices (con’t) • Place a blank line between the last variable declaration and the first executable statement of the program. • Indent the body of the program 3 to 5 tab stops -be consistent! CMSC 104, Lecture 12 22
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